$\begingroup$While a neutron star wouldn't be, it's an interesting question whether in theory, pure neutrons would be, not that there would be much use for a transparent few miles across, super dense object that would bend light around it and you couldn't get close to anyway, but it's a fun question. Likely such an object, made of pure Neutrons, the outer layers would degenerate into protons anyway, so, transparency wouldn't be possible.$\endgroup$
– userLTKApr 8 '15 at 0:25

4 Answers
4

Neutron stars are one of the least transparent objects in the universe. There are still lots of protons and free electrons, and most importantly the density of these charged species is extremely high. Also, the quarks, which constitute neutrons, are charged---so due to their density, I would guess (hopefully an expert will chime in) that that is also plenty to keep them optically thick. Definitely once you get to gamma-ray energies, neutrons will interact with photons.

$\begingroup$+1 for pointing out that neutron stars don't just consist of neutrons.$\endgroup$
– Rob JeffriesJan 14 '15 at 18:09

$\begingroup$Concerning quarks: If quarks are presented they are most probably in a superconducting state. Some of the most popular candidates, the CFL phase is actually nearly transparent due to the formation pattern of the Cooper pairs$\endgroup$
– NoldigFeb 19 '19 at 19:03

$\begingroup$Hi Jerry, I was wondering about this while formulating my response. Would you need to worry about the magnetic field before it's energy density became comparable to that of the radiation field? Or is that the relevant regime?$\endgroup$
– DilithiumMatrixMar 22 '13 at 4:03

1

$\begingroup$In principle the gamma could, indeed, couple pretty well to the neutron EDM, but in practice the vast majority of those neutrons have no accessible states to scatter into---you have to get them above the Fermi surface, right? I think that Ron and John's analyses in the proposed duplicate are the way to go.$\endgroup$
– dmckee --- ex-moderator kitten♦Mar 22 '13 at 14:03

$\begingroup$@dmckee: I'm sure you could work out what the scattering depth of a solid substance with a constant magnetic polarization. And I'm pretty sure that it would be pretty shallow for something like a neutron star, which is basically a ferromagnet.$\endgroup$
– Jerry SchirmerMar 23 '13 at 0:34

You can think about it in more practical terms. It is known that one metre thick lead can stop very energetic $\gamma$-photons. Then, given that a neutron star is billions of times denser than lead and several kilometres across, it is not very likely that a neutron star will be transparent.

$\begingroup$@dmckee If you take into account the fact that there are some $10^{44}$ neutrons per m$^3$(only a rough estimate,) about 16 orders of magnitude higher than the number of electrons in lead in the same volume, and that each neutron has three charged particles (quarks,) then perhaps the answer maybe not such suspect. If the question is talking about transparency to neutrinos, then the answer could be different, as it could be possible for neutrinos to travel through the neutron star.$\endgroup$
– JKLMar 22 '13 at 23:29

$\begingroup$A visible photon can not interact with the bulk of those neutrons because they are far, far below the Fermi surface. The bulk material is presumably opaque anyway simply because the conversion of proton to neutrons only proceeds until the electron Fermi surface is in agreement with that of the neutrons, so there are free protons in the mix.$\endgroup$
– dmckee --- ex-moderator kitten♦Mar 22 '13 at 23:39

$\begingroup$@dmckee if you are still around, could you explain the "Fermi surfaces in agreement"?$\endgroup$
– JDługoszJan 14 '15 at 11:21

$\begingroup$@jdlugosz By "in agreement" I mean at the same energy. Of course, that is only helpful if you understand what "Fermi surface" implies. The reason for this is that converting a proton requires energy to get the neutron above the neutron Fermi surface, but removes an electron from the electron Fermi surface.$\endgroup$
– dmckee --- ex-moderator kitten♦Jan 14 '15 at 14:16

Light with wavelength $\lambda\gg10\,\mathrm{km}$ can pass around a neutron star thanks to diffraction, even if the star is made of a perfectly absorbing material. That's exceptionally low-energy radio waves, though, and pretty different from what you probably had in mind.